The issue here turned out to be that I was not scrubbing the write-protect
screw hole enough. I had to take the board out and scrub the screw hole
with rubbing alcohol in order to get enough traction to clean the gunk I
wasn't seeing. I had been scrubbing the board while leaving it in the
case; apparently that wasn't enough. I finally got a success on flashrom
--wp-disable after the fifth or sixth time cleaning. Thanks for the help!